Carbon dioxide is a colorless gas, which was first recognized in 1577 by Van Helmont who detected it in the by-products of both fermentation and charcoal burning. CO2 is used in solid (dry ice), liquid and gaseous form in a variety of industrial applications such as beverage carbonation, welding and chemicals manufacture. It occurs in the products of combustion of all carbonaceous fuels and can be recovered from them in a variety of ways. CO2 is widely used today as a by-product of synthetic ammonia production, fermentation, lime kiln operations, and from flue gases by absorption processes. CO2 is also a product of animal metabolism and is critically important in the life cycles of both animals and plants. CO2 is present in our earth's atmosphere in small quantities (0.03%, by volume).
Carbon dioxide (CO2) will extinguish fires in almost all combustibles except for a few active metals, metallic salts and substances containing oxygen, i.e., nitrates, chlorates.
The advantages of carbon dioxide gas for fire extinguishing purposes have been long known. As early as 1914, the Bell Telephone Company of Pennsylvania installed a number of seven pound capacity portable CO2 extinguishers for use on electrical wiring and equipment. By the 1920s, automatic systems utilizing carbon dioxide were available. In 1928, work on the NFPA Standard for carbon dioxide extinguishing systems was begun.
Over the years, two methods of applying carbon dioxide have been developed. The first technique is the total flooding application. The total flooding technique consists of filling an enclosure with carbon dioxide vapor to a prescribed concentration. This technique is applicable both for surface-type fires and potential deep-seated fires. For surface-type fires, such as would be expected with liquid fuels, a minimum concentration of 34% carbon dioxide by volume is mandated. Considerable test work has been done with carbon dioxide on liquid fuels and appropriate minimum design concentrations have been arrived at for a large number of common liquid fire hazards. This method of application has limitations in the amount and distance of applied CO2 that can be effectively delivered. This leads to a small, effective coverage area for such application.
For deep-seated type hazards the minimum permitted concentration is 50% carbon dioxide by volume. Fifty percent design concentration is used for hazards involving electrical gear, wiring insulation, motors, and the like. For hazards involving record storage, such as bulk paper, a 65% concentration of carbon dioxide is required. For substances such as fur and bag-house type dust collectors, a 75% concentration of CO2 is mandated. It should be noted that most surface burning and open flaming will stop when the concentration of CO2 in the air reaches about 20% or less. Thus, it should be apparent that a considerable factor of safety is built in to these minimum CO2 concentrations required by the Standard. Flame extinguishment has typically not been considered to be sufficient fire protection by those who developed the CO2 Standard. This is in contrast to the guidelines given in standards for other gaseous extinguishing agents. Some of these standards may mandate agent concentrations which may be sufficient to extinguish open flame but will not produce a truly inert atmosphere.
The other method of application which has been developed for carbon dioxide is referred to as local application. Local application systems are appropriate only for the extinguishment of surface fires in flammable liquids, gases and very shallow solids where the hazard is not enclosed or where the enclosure of the hazard is not sufficient to permit total flooding. Hazards such as dip tanks, quench tanks, spray booths, printing presses, rolling mills, and the like can be successfully protected by a local application type system. In this system, the discharge of CO2 is directed at the localized fire hazard. The entire fire hazard area is then blanketed in CO2 without actually filling the enclosure to a predetermined concentration.
Extinguishers have been considered a first line of defense in fighting fires. Their practical and functional use tends to render them ideal as a means of prevention and protection against all types of fires. However, the common fire extinguisher typically has only a 3-6 foot range and may have both clean-up problems and high costs. Large commercial CO2 foam solutions to fight fires tend to be expensive in more ways than one. Due to cost, effective coverage area, and safety distance requirements, the local application of CO2 may have limitations in proper fire containment and extinguishing.
FIG. 1 shows a fire tetrahedron. The image shown is known to fire fighters as the fire tetrahedron it may be used to better understand the properties of fire and extinguishment techniques.
It is very similar to the fire triangle which does not represent the chemical chain reaction. The fire tetrahedron is based on the components of extinguishing a fire. Each component represents a property of flaming fire; fuel 11, oxygen 12, heat 13, and chemical chain reaction 14. Extinguishment is based upon removing or hindering any one of these properties. The most common property to be removed is heat. Heat is commonly eliminated by using water. Water is used because it absorbs heat extremely well and is cost efficient. During fire operations you may see objects being placed outside a structure. Though this is commonly referred to as salvage operations, it also acts to remove any fuel from the fire. Without the objects exposed to heat there can be no flammable gasses given off to burn. The third property, oxygen, is usually the hardest to remove. Oxygen removal is typically accomplished when a carbon dioxide extinguisher is used on a fire. In more extreme cases explosives may be used on a fire. The explosion will use up the oxygen in the immediate area. Finally, the last property is the chemical chain reaction. This can be considered the reaction of the reducing agent (fuel) with the oxidizing agent (oxygen). An example of an extinguishment method by hindering the chemical chain reaction is Halon or FM200 extinguishers.
With a surface-type fire, that is, a fire which has not heated the fuel to its auto-ignition temperature much beyond the very surface of that fuel, extinguishment is rapid. Such surface fires are usually the case when liquid fuels are involved. Unfortunately, there is no guarantee that all hazards will produce surface fires. In fact, a great many hazards are more likely to produce fires which will penetrate for some depth into the fuel. Such fires are commonly referred to as deep-seated. When dealing with a so-called deep-seated potential, it is necessary not only to remove the oxygen and decrease the gaseous phase of the fuel in the area, but it may be equally important to permit the heat which is built up in the fuel itself to dissipate. If the heat is not dissipated and the inert atmosphere is removed, the fire may very easily re-flash. For such hazards, it is often necessary to reduce the concentration of oxygen and gaseous fuel to a point where not only is the open flaming stopped, but also any smoldering is eliminated. To accomplish this, the concentration of agent should be held for a sufficiently long time to permit adequate dissipation of built-up heat. The NFPA Standard 12 on carbon dioxide systems has long been a leader in prescribing thorough and conservative fire protection.